The present invention relates to a variable displacement compressor used in a refrigeration circuit that performs heat exchange at temperatures both above and below the critical temperature of the refrigerant. Specifically, the present invention pertains to a method and an apparatus for controlling a variable displacement compressor that changes its displacement based on the difference between a control pressure in a control chamber and a suction pressure in a suction pressure zone.
A variable displacement compressor used in a refrigeration circuit generally has a housing that houses a control chamber and a rotatable drive shaft. Cylinder bores extend through a cylinder block, which forms part of the housing. A piston is reciprocally retained in each cylinder bore. A swash plate is tiltably supported on the drive shaft in the control chamber. The swash plate converts rotation of the drive shaft into reciprocation of the pistons. This draws refrigerant gas into the associated cylinder bore from a suction chamber, compresses the refrigerant gas, and then discharges the compressed refrigerant gas into a discharge chamber. The inclination of the swash plate is altered in accordance with the difference between the pressure of the cylinder bores and the pressure of the control chamber. In other words, the swash plate's inclination is altered in accordance with the difference between the suction pressure and the control pressure. The inclination of the swash plate is smaller when the pressure difference is larger. That is, the inclination of the swash plate decreases as the control pressure becomes higher relative to the suction pressure. A decrease in the inclination of the swash plate shortens the stroke of the pistons and decreases the displacement of the compressor.
A typical refrigeration circuit having the above compressor further includes a condenser, an expansion valve and an evaporator. The compressor compresses gaseous refrigerant sent from the evaporator. The condenser receives high pressure, high temperature gaseous refrigerant from the compressor. The condenser then cools the refrigerant by performing heat exchange with the outside air thereby liquefying the refrigerant. The expansion valve receives the liquefied refrigerant from the condenser and expands the refrigerant into low temperature, low pressure mist. The evaporator gasifies the refrigerant mist by performing heat exchange between the refrigerant and air to be sent to the passenger compartment.
A typical refrigeration circuit uses chlorofluorocarbon as its refrigerant. However, Japanese Unexamined Patent Publication No. 8-110104 describes a compressor that employs carbon dioxide (CO.sub.2) as its refrigerant. The critical temperature of carbon dioxide is thirty-one degrees centigrade, which is about twenty degrees lower than that of chlorofluorocarbon. In a refrigeration circuit using chlorofluorocarbon as the refrigerant, the condenser cools chlorofluorocarbon refrigerant to temperatures below the critical temperature of the chlorofluorocarbon. However, in a refrigeration circuit using carbon dioxide as the refrigerant, the carbon dioxide can be cooled in a temperature range higher than the critical temperature of carbon dioxide especially in summer, when the outside temperature is high.
A refrigeration circuit that uses chlorofluorocarbon as its refrigerant includes a temperature-type expansion valve. When the speed of the compressor's drive shaft increases while the thermal load applied on the circuit remains constant, the compressor increases the amount of refrigerant discharged therefrom. This increases the flow rate of the chlorofluorocarbon refrigerant in the circuit and prevents the evaporator from performing sufficient heat exchange. Accordingly, the degree of superheating of the chlorofluorocarbon refrigerant decreases at the outlet of the evaporator. The temperature-type expansion valve reduces the flow rate of chlorofluorocarbon refrigerant supplied to the evaporator in accordance with a decrease of the degree of superheating. The reduction of refrigerant flow rate allows the evaporator to perform sufficient heat exchange. As a result, the degree of superheating is maintained at a proper level. Consequently, the pressure of refrigerant supplied from the evaporator to the compressor is lowered. That is, the suction pressure is lowered. A decrease of the suction pressure results in a greater difference between the suction pressure and the control pressure, which, in turn, decreases the compressor displacement. The decrease of the displacement maintains the refrigerant performance of the refrigeration circuit. The decrease of the suction pressure also lowers the evaporating temperature of the chlorofluorocarbon refrigerant. Thus, the compressor can be optimally controlled in accordance with fluctuations in its suction pressure by referring to the temperature or the pressure of the refrigerant at the outlet of the evaporator.
In a refrigeration circuit using carbon dioxide as the refrigerant, the condenser can cool carbon dioxide refrigerant in a temperature range above the critical temperature of carbon dioxide. This indicates that the pressure of the refrigerant in the condenser changes in accordance with thermal load applied to the refrigeration circuit even if the temperature of the refrigerant in the condenser is the same. Thus, a carbon dioxide type refrigeration circuit includes a pressure-type expansion valve. The pressure-type expansion valve controls the flow rate of refrigerant in accordance with the temperature and pressure of refrigerant in the condenser, or the temperature and the pressure of refrigerant discharged from the compressor.
For example, an increase in the speed of the compressor's drive shaft with a constant thermal load acting on the refrigeration circuit raises the pressure of refrigerant discharged from the compressor, or discharge pressure. However, a pressure-type expansion valve increases the flow rate of carbon dioxide refrigerant supplied to the evaporator as the discharge pressure increases, which prevents the suction pressure of the compressor from dropping quickly. Thus, the displacement of the compressor is not decreased immediately. Accordingly, the refrigerant performance of the circuit is not adjusted promptly. Further, the evaporating temperature of the carbon dioxide refrigerant in the evaporator is not quickly lowered. Therefore, it is difficult to optimally control the compressor in accordance with fluctuations of the suction pressure by referring to the temperature and the pressure of the carbon dioxide refrigerant at the outlet of the evaporator. Thus, the use of a pressure-type expansion valve in a carbon dioxide refrigeration circuit increases unnecessary operation of the compressor thereby increasing the power consumption of the compressor and the load acting on the compressor.